In devices involving the amplification of sound, or the provision of electrical stimuli so as to produce a sound percept, it is necessary to utilise an AGC system. This adjusts the gain utilised in an amplifier, so as to take account of the prevailing sound level in the vicinity. The use of a simple AGC which, say, always adjusts to a predefined level can give rise to difficulties where no meaningful sound is currently being produced--in this case, background noise may be amplified so as to become intrusive. In a conventional system with no manual gain control, this background noise will be amplified so as to have an average level equivalent to that of the sounds of interest, such as speech. This is clearly unsatisfactory for the user.
In most practical systems, a sensitivity or input gain control is provided to allow the maximum AGC gain to be varied by the user to an appropriate level. Patients will generally adjust this to a position which maximises the perceived signal to noise ratio. This will generally be where the AGC is just on the point of operating when another person is speaking to them. A higher setting than this means that the background noise level is raised by the AGC in the absence of speech whilst the signal amplitude is lowered when speech is present. This has the adverse effect that the background noise level appears to be higher, and will tend to influence adversely speech discrimination, as the signal amplitude information is compressed. If, on the other hand, the gain is set too low, the AGC will not operate at all and the signal may well be too soft to allow for good speech discrimination by the user.
Notwithstanding these difficulties, AGC is generally provided, as when the user himself speaks, the signal perceived by the microphone will be substantially louder than when another person speaks. Further, the person speaking may raise his or her voice. Without an AGC, either of these cases leads to limiting and distortion of the signal and corresponding degradation of output from the signal processor.
A further difficulty arises when a user moves from a quiet situation to a noisier one. In this situation, the noise floor is higher and the conversation louder. The AGC operates when other people and the user are speaking and the perceived signal to noise ratio is worse than the actual ratio because during speech the gain is lower than in the breaks. In this case, it would be advantageous to lower the sensitivity.
U.S. Pat. No. 4,718,099 to Hotvet discloses an AGC arrangement in which if an input signal exceeds a predetermined maximum, then a fast attack response acts to reduce gain. A variable release period is used. However, there is no disclosure of establishing a noise floor for the arrangement--an averaging arrangement is used. Another arrangement using a noise averaging and variable attack time approach is disclosed in U.S. Pat. No. 5,144,675 to Killion et al. Other disclosures use a combination of long and short term averaging, and a difference amplifier--for example U.S. Pat. No. 4,996,712 to Lawrence et al, and Australian patent application No 17099/88 by Hughes Aircraft Company.
It is an object of the present invention to provide an improved AGC system for hearing aids and auditory prostheses such that the sensitivity of the system responds better to the prevailing sound environment than previous devices.